MMap certain files, leave the rest to the regular dir

[Jason Rutherglen]

On the topic of MMaping files. Would a Directory implementation
that transparently MMaps only certain files be interesting? It
could MMap files that are accessed frequently (term dict,
postings), as opposed to files such as docstores that are
accessed less frequently. This could be built using LUCENE-1618
so maybe this idea is just a benchmark.